(1) Field of the Invention
The present invention relates to nucleating agents for crystallized glasses, crystallized glasses, magnetic disc substrates and magnetic discs.
(2) Related Art Statement
Magnetic disc substrates made of crystallized glasses have recently been investigated. In the crystallized glass, almost all alkali metal ions contained are present in the crystalline phase, whereas a very small amount of alkaline metal ions exist in the glass matrix. Accordingly, a problem that a magnetic film is corroded with an alkali metal component dissolves out.
With the development in the mass media, a demand for recording a large volume of information, particularly, such as image information, in a more miniaturized magnetic disc has been increasing, so that further improvement on the recording density of the magnetic discs has been sought. As a result, it has been demanded that the center line average height (Ra) be reduced to a level of not more than 10 xc3x85 especially in the read/write zone of the magnetic disc.
However, hardness of the crystalline phase differs from that of the amorphous phase in the crystallized glass. Owing to this, fine uneven portions are inevitably formed between the crystalline phase and the amorphous phase even after polishing. Consequently, it has been difficult to reduce the center line average height of the worked surface of the crystallized glass to not more than 10 xc3x85. Particularly with respect to a magnetic disc substrate made of Li2Oxe2x80x94Al2O3xe2x80x94SiO2 based crystallized glass, it is extremely difficult to satisfy the above requirement. Although the center line average height of the non-crystallized glass or glass, on the way of crystallization, having a low crystallization degree will be able to be reduced to not more than 10 xc3x85 after fine polishing, such materials have relatively low strength.
In JP-A 9-208,260, the applicant tried to obtain a magnetic disc substrate having the surface roughness of 2 to 10 xc3x85 by using a magnetic disc substrate body made of a crystallized glass with a specific composition and finely polishing the substrate body. NGK insulators, Ltd. discovered that the center line average height of the finely polished surface can be reduced to a level of 20 xc3x85 or less if a specific Li2Oxe2x80x94Al2O3xe2x80x94SiO2 based crystallized glass is used, and concretely disclosed the magnetic disc substrate using this crystallized glass in JP-A-9-35234.
The present inventors"" investigation clarified that it is advantageous to precipitate a lithium disilicate and a cristobalite phase in the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 based crystallized glass so as to control the coefficient of thermal expansion thereof in an appropriate range and give a magnetic disc substrate having high strength.
However, since particularly the cristobalite phase is likely to cohere during the crystallization, the cohered particle diameter may reach as much as 0.5 xcexcm. If a considerable amount of the cristobalite phase is produced in the crystallized glass and coexists with the lithium disilicate there, it causes a problem that it is difficult to proceed with working the crystallized glass, because the hardness of the cristobalite phase is higher than that of the lithium disilicate and higher than that of the matrix of the amorphous phase. That is, since the cristobalite coheres, it was difficult to reduce the center line average height after finely polishing the magnetic disc substrate to a lower level, for example, to a level of 10 xc3x85 or less.
It is an object of the present invention to prevent cohesion of the cristobalite and make particles in the cristobalite in the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 based crystallized glass when the lithium disilicate phase and the cristobalite phase are precipitated.
It is another object of the present invention to provide a nucleating agent which promotes the dispersion of the cristobalite phase in the crystallized glass.
The nucleating agent according to the present invention is to make fine particles at least in the cristobalite phase when added into the crystallized glass containing at least the cristobalite phase, and comprises one or more kinds of metallic compounds selected from the group consisting of molybdenum compounds and tantalum compounds.
Further, the present invention relates to an Li2Oxe2x80x94Al2O3xe2x80x94SiO2 based crystallized glass comprising lithium disilicate, Li2Oxe2x80x942SiO2, as a main crystalline phase, a cristobalite phase as a sub-crystalline phase, and one or more kinds of metallic oxides selected from the group consisting of molybdenum oxide and tantalum oxide in a total amount of 0.5 to 7.0% by weight.
The present invention also relates to a magnetic disc substrate made of the above crystallized glass, which has a flat and smooth surface with a center line average height (Ra) of not more than 10 xc3x85, preferably not more than 8 xc3x85, and more preferably not more than 6 xc3x85 or not more than 5 xc3x85.
The present invention further relates to a magnetic disc comprising the above magnetic disc substrate, an under film formed on the flat and smooth surface of the substrate, and a magnetic metallic layer formed on the under film.
The present inventors discovered that when compounds producing molybdenum oxide and tantalum oxide, respectively, are incorporated into a crystallized glass having such a composition as being capable of producing a cristobalite phase after thermal treatment, the compounds function as an auxiliary nucleating agent, so that the dispersion of the cristobalite is prompted and made finer.
This nucleating agent was suitable particularly for the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 based crystallized glass comprising lithium disilicate, Li2Oxe2x80x942SiO2 and the cristobalite as a main crystalline phase. As a result, its center-line average height could be reduced to not more than 6 xc3x85 or not more than 5 xc3x85, for example, by finely polishing the surface of the crystallized glass.
These and other objects, features and advantages of the invention will be apparent from the following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes could be easily made by the skilled person in the art to which the invention pertains.